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<html>
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<title>17.5.Write Ahead Log</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="runtime-config-wal"></a>17.5.Write Ahead Log</h2></div></div></div>
<p> See also <a href="wal-configuration.html" title="26.3.WAL Configuration">Section26.3, “<acronym class="acronym">WAL</acronym> Configuration”</a> for details on WAL
tuning.
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="runtime-config-wal-settings"></a>17.5.1.Settings</h3></div></div></div>
<div class="variablelist"><dl>
<dt>
<a name="guc-fsync"></a><span class="term"><code class="varname">fsync</code> (<code class="type">boolean</code>)</span>
</dt>
<dd>
<p> If this option is on, the <span class="productname">PostgreSQL</span> server
will try to make sure that updates are physically written to
disk, by issuing <code class="function">fsync()</code> system calls or various
equivalent methods (see <a href="runtime-config-wal.html#guc-wal-sync-method">wal_sync_method</a>).
This ensures that the database cluster can recover to a
consistent state after an operating system or hardware crash.
</p>
<p> However, using <code class="varname">fsync</code> results in a
performance penalty: when a transaction is committed,
<span class="productname">PostgreSQL</span> must wait for the
operating system to flush the write-ahead log to disk. When
<code class="varname">fsync</code> is disabled, the operating system is
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
However, if the system crashes, the results of the last few
committed transactions may be lost in part or whole. In the
worst case, unrecoverable data corruption may occur.
(Crashes of the database software itself are <span class="emphasis"><em>not</em></span>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
</p>
<p> Due to the risks involved, there is no universally correct
setting for <code class="varname">fsync</code>. Some administrators
always disable <code class="varname">fsync</code>, while others only
turn it off during initial bulk data loads, where there is a clear
restart point if something goes wrong. Others
always leave <code class="varname">fsync</code> enabled. The default is
to enable <code class="varname">fsync</code>, for maximum reliability.
If you trust your operating system, your hardware, and your
utility company (or your battery backup), you can consider
disabling <code class="varname">fsync</code>.
</p>
<p> This option can be set at server start or in the
<code class="filename">postgresql.conf</code> file. If you turn
this option off, also consider turning off
<a href="runtime-config-wal.html#guc-full-page-writes">full_page_writes</a>.
</p>
</dd>
<dt>
<a name="guc-wal-sync-method"></a><span class="term"><code class="varname">wal_sync_method</code> (<code class="type">string</code>)</span>
</dt>
<dd>
<p> Method used for forcing WAL updates out to disk.
If <code class="varname">fsync</code> is off then this setting is irrelevant,
since updates will not be forced out at all.
Possible values are:
</p>
<div class="itemizedlist"><ul type="disc">
<li><p> <code class="literal">open_datasync</code> (write WAL files with <code class="function">open()</code> option <code class="symbol">O_DSYNC</code>)
</p></li>
<li><p> <code class="literal">fdatasync</code> (call <code class="function">fdatasync()</code> at each commit)
</p></li>
<li><p> <code class="literal">fsync_writethrough</code> (call <code class="function">fsync()</code> at each commit, forcing write-through of any disk write cache)
</p></li>
<li><p> <code class="literal">fsync</code> (call <code class="function">fsync()</code> at each commit)
</p></li>
<li><p> <code class="literal">open_sync</code> (write WAL files with <code class="function">open()</code> option <code class="symbol">O_SYNC</code>)
</p></li>
</ul></div>
<p> Not all of these choices are available on all platforms.
The default is the first method in the above list that is supported.
This option can be set at server start or in the
<code class="filename">postgresql.conf</code> file.
</p>
</dd>
<dt>
<a name="guc-full-page-writes"></a><span class="term"><code class="varname">full_page_writes</code> (<code class="type">boolean</code>)</span>
</dt>
<dd>
<p> When this option is on, the <span class="productname">PostgreSQL</span> server
writes the entire content of each disk page to WAL during the
first modification of that page after a checkpoint.
</p>
<p> This parameter is currently ignored (treated as always <code class="literal">on</code>)
because turning it off can cause failure to recover from crashes
even when no hardware or OS-level error occurred. This will be
fixed in some future release, or else the parameter will be removed
entirely.
</p>
</dd>
<dt>
<a name="guc-wal-buffers"></a><span class="term"><code class="varname">wal_buffers</code> (<code class="type">integer</code>)</span>
</dt>
<dd>
<p> Number of disk-page buffers allocated in shared memory for WAL data.
The default is 8. The setting need only be large enough to hold
the amount of WAL data generated by one typical transaction, since
the data is written out to disk at every transaction commit.
This option can only be set at server start.
</p>
<p> Increasing this parameter may cause <span class="productname">PostgreSQL</span>
to request more <span class="systemitem">System V</span> shared
memory than your operating system's default configuration
allows. See <a href="kernel-resources.html#sysvipc" title="16.4.1.Shared Memory and Semaphores">Section16.4.1, “Shared Memory and Semaphores”</a> for information on how to
adjust those parameters, if necessary.
</p>
</dd>
<dt>
<a name="guc-commit-delay"></a><span class="term"><code class="varname">commit_delay</code> (<code class="type">integer</code>)</span>
</dt>
<dd><p> Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay can allow multiple transactions to be committed
with only one <code class="function">fsync()</code> system call, if
system load is high enough that additional transactions become
ready to commit within the given interval. But the delay is
just wasted if no other transactions become ready to
commit. Therefore, the delay is only performed if at least
<code class="varname">commit_siblings</code> other transactions are
active at the instant that a server process has written its
commit record. The default is zero (no delay).
</p></dd>
<dt>
<a name="guc-commit-siblings"></a><span class="term"><code class="varname">commit_siblings</code> (<code class="type">integer</code>)</span>
</dt>
<dd><p> Minimum number of concurrent open transactions to require
before performing the <code class="varname">commit_delay</code> delay. A larger
value makes it more probable that at least one other
transaction will become ready to commit during the delay
interval. The default is five.
</p></dd>
</dl></div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="runtime-config-wal-checkpoints"></a>17.5.2.Checkpoints</h3></div></div></div>
<div class="variablelist"><dl>
<dt>
<a name="guc-checkpoint-segments"></a><span class="term"><code class="varname">checkpoint_segments</code> (<code class="type">integer</code>)</span>
</dt>
<dd><p> Maximum distance between automatic WAL checkpoints, in log
file segments (each segment is normally 16 megabytes). The
default is three. This option can only be set at server start
or in the <code class="filename">postgresql.conf</code> file.
</p></dd>
<dt>
<a name="guc-checkpoint-timeout"></a><span class="term"><code class="varname">checkpoint_timeout</code> (<code class="type">integer</code>)</span>
</dt>
<dd><p> Maximum time between automatic WAL checkpoints, in
seconds. The default is 300 seconds. This option can only be
set at server start or in the <code class="filename">postgresql.conf</code>
file.
</p></dd>
<dt>
<a name="guc-checkpoint-warning"></a><span class="term"><code class="varname">checkpoint_warning</code> (<code class="type">integer</code>)</span>
</dt>
<dd><p> Write a message to the server log if checkpoints caused by
the filling of checkpoint segment files happen closer together
than this many seconds (which suggests that
<code class="varname">checkpoint_segments</code> ought to be raised). The default is
30 seconds. Zero disables the warning.
</p></dd>
</dl></div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="runtime-config-wal-archiving"></a>17.5.3.Archiving</h3></div></div></div>
<div class="variablelist"><dl>
<dt>
<a name="guc-archive-command"></a><span class="term"><code class="varname">archive_command</code> (<code class="type">string</code>)</span>
</dt>
<dd>
<p> The shell command to execute to archive a completed segment of
the WAL file series. If this is an empty string (the default),
WAL archiving is disabled. Any <code class="literal">%p</code> in the string is
replaced by the absolute path of the file to archive, and any
<code class="literal">%f</code> is replaced by the file name only. Use
<code class="literal">%%</code> to embed an actual <code class="literal">%</code> character in the
command. For more information see <a href="backup-online.html#backup-archiving-wal" title="23.3.1.Setting up WAL archiving">Section23.3.1, “Setting up WAL archiving”</a>. This option can only be set at
server start or in the <code class="filename">postgresql.conf</code>
file.
</p>
<p> It is important for the command to return a zero exit status if
and only if it succeeds. Examples:
</p>
<pre class="programlisting">archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" /mnt/server/archivedir/"%f"' # Windows</pre>
<p>
</p>
</dd>
</dl></div>
</div>
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